ok, reinstalling MPLab did the trick. The program compiled and uploaded fine, except for these warnings from the compiler:

Warning[230] C:\PBP\18F452.INC 20 : __CONFIG has been deprecated for PIC18 devices. Use directive CONFIG.
Warning[230] C:\PBP\18F452.INC 21 : __CONFIG has been deprecated for PIC18 devices. Use directive CONFIG.
Warning[230] C:\PBP\18F452.INC 22 : __CONFIG has been deprecated for PIC18 devices. Use directive CONFIG.

But, whatever they mean, it works! Thanks for the help!